(1) Field of the Invention
The present invention relates to a method for purifying an impure aqueous hydrogen peroxide solution to obtain a highly purified aqueous hydrogen peroxide solution which contains each metal cation, each anion and organic impurities in a very small amount, respectively.
(2) Description of Related Art
Aqueous hydrogen peroxide solutions are in wide use not only as a reagent but also in many fields such as bleaching, chemical polishing and the like. In recent years, the utilization of an aqueous hydrogen peroxide solution has spread to the field of semiconductor production where said solution is used for the cleaning, etching, etc. of water; in this connection, an aqueous hydrogen peroxide solution of high purity has become necessary.
That is, in the production of a semiconductor, particularly VLSI, its memory capacity and number of elements have increased and, in parallel therewith, its dimension of minimum pattern has become very fine. When the aqueous hydrogen peroxide solution which is used for the cleaning of VLSI contains impurities and fine particles, the particles adhere onto the surface film of the VLSI, causing a thinning of the oxide film or a
25 deterioration of the voltage resistance of the SiO.sub.2 film. Further when impurities diffuse onto the Si substrate, there occurs an increase in leakage current and a shortening of carrier life, causing a decrease in yield of product LSI and a deterioration of properties of said LSI.
Thus, in the production of a semiconductor, it is required to develop an aqueous hydrogen peroxide solution of high purity which contains fine particles and impurities in a very small amount.
As mentioned above, in the field of semiconductor production, the chemical agents used therefor are desired to contain impurities in as small an amount as possible because the impurities contained in said agents greatly affect the quality of the product obtained. In particular, the aqueous hydrogen peroxide solution used for cleaning a semiconductor is required to have a very high purity and to contain each metal cation in an amount of not more than 5 ppb (.mu.g/kg), each anion in an amount of not more than 10 ppb and organic impurities in an amount of not more than 5 ppm (mg/kg) in terms of total organic carbon amount.
Meanwhile, hydrogen peroxide is currently produced in industry mainly by the autoxidation of anthrahydroquinone (this process is hereinafter referred to as the anthraquinone process). The aqueous hydrogen peroxide solution produced by the anthraquinone process contains a very small amount of impurities, and the amount is ordinarily about 10-500 mg/l in terms of total organic carbon amount.
Said aqueous hydrogen peroxide solution further contains, as impurities, ordinarily about 5 ppb to about 10 ppm of each of various metal cations and about 10 ppb to about 10 ppm of each of various anions as a result of dissolution of construction materials of production and storage facilities, addition of additives, etc.
In order to remove these impurites to obtain a purified aqueous hydrogen peroxide solution, there have conventionally been known several methods as shown below, and some of them are employed in actual use.
(a) Japanese Patent Publication No. 3816/1953
This patent document discloses a method comprising contacting an aqueous hydrogen peroxide solution with a sulfonated aromatic hydrocarbon type cation exchange resin to remove mainly the metal cations contained in said aqueous hydrogen peroxide solution.
(b) U.S. Patent No. 3297404
This patent document discloses a method for removing the anion impurities contained in an aqueous hydrogen peroxide solution by using an anion exchange resin having a quaternary ammonium group of carbonate and/or bicarbonate form.
(c) Japanese Patent Publication No. 26095/1971
This patent document discloses a method for removing by adsorption the organic impurities contained in an aqueous hydrogen peroxide solution by using a resin having a reticulate molecular structure obtained by crosslinking with divinylbenzene but having no ion exchange group.
(d) Japanese Laid-Open Patent Publication No. 156004/1988
This patent document discloses a method for removing by adsorption the organic impurities contained in an aqueous hydrogen peroxide solution by using a halogen-containing porous resin having a true specific gravity of 1.1-1.3.
Any of the above known methods for purification of an aqueous hydrogen peroxide solution, however, is unable to provide an aqueous hydrogen peroxide solution of such high quality as to allow its use in the production of a semiconductor, particularly LSI. That is, any of the above known methods cannot provide a high purity aqueous hydrogen peroxide solution containing each metal cation in an amount of no t more than 5 ppb, each anion in an amount of not more than 10 ppb and organic impurities in an amount of not more than 5 ppm.
For example, in the method using a cation exchange resin, metal cation impurities can be removed but anion impurities increase because a part of the SO.sub.3 H group which is the ion exchange group of said resin is deteriorated by the action of hydrogen peroxide and dissolves in the form of SO.sub.4.sup.2-.
Also, in the method using a halogen-containing porous resin, organic impurities can be advantageously removed but anion impurities increase because a part of the halogen in said resin dissolves.
There have also been proposed methods for purifying an aqueous hydrogen peroxide solution, which use a plurality of ion exchange resins in combination. For example, Poland Patent No. 55,378 discloses a method comprising passing an aqueous hydrogen peroxide solution through a cation exchange resin, an anion exchange resin and a cation/anion mixed-bed resin in this order to contact the solution with each resin. This method, however, is unable to remove organic impurities.
Thus, in any of the conventionally known methods, it has been impossible to obtain a high quality aqueous hydrogen peroxide solution containing each metal cation, each anion and organic impurities in respective amounts of not more than the above mentioned levels.